Spectral resolution of fluxional organometallics. The observation and FTIR characterization of all-terminal [Rh 4(CO) 12]

Abstract

In situ FTIR spectroscopy at 1 cm -1 resolution was conducted on n-hexane solutions of the bridged [Rh 4(CO) 9-(μ-CO) 3] in the interval T = 268-288 K and P T = 0.1-7.0 MPa using either helium or carbon monoxide as dissolved gas. Analysis of the spectral data sets was conducted using band-target entropy minimization (BTEM), in order to recover the pure component spectra. A new spectral pattern was recovered with terminal vibrations at 2075, 2069.8, 2044.6 and 2042 cm -1. The new spectrum is consistent with an all-terminal [Rh 4(CO) 12] species with a C 3v anticubeoctahedron structure where 2 different [Rh(CO) 3] moieties exist, although the presence of some T d structure can not be entirely excluded. The equilibrium between all-terminal [Rh 4(CO) 12] and the bridged [Rh 4(CO) 9(μ-CO) 3] was determined in the presence of both helium and CO. The equilibrium constant K eq = [Rh 4(CO) 12]/[Rh 4(CO) 9-(μ-CO) 3] at 275 K was ca. 0.011 and the determined equilibrium parameters were Δ rG = 12.63 ± 4.8 kJ mol -1, Δ rH = -21.45 ± 2.3 kJ mol -1 and Δ rS = -114.3 ± 8.35 J mol -1 K -1. The free energy indicates a very small difference between the bridged and terminal geometry, and the lower entropy is consistent with a higher symmetry. This finding helps to address a long-standing issue concerning the existence of various [M 4(CO) 12] symmetries. In a more general context, the present study illustrates the considerable utility of quantitative infrared spectroscopy (occurring on a fast vibrational timescale) combined with sophisticated deconvolution techniques in order to resolve systems which have been demonstrated to be fluxional on the NMR timescale. © The Royal Society of Chemistry 2005.